In at least one conventional computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam, which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the “imaging plane”. The x-ray beam passes through a medical patient or other imaging object. The x-ray beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated x-ray beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam passing through the imaging object, such as the medical patient's body. Each detector element of the array produces a separate electrical signal that is a measurement of the x-ray beam's attenuation at the detector location. Separate attenuation measurements from all the detectors are acquired to produce a transmission profile.
In conventional third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around an imaging object so that the angle at which the x-ray beam intersects the imaging object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two-dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
At least one known CT imaging system is available that combines a gantry rotation rate of 0.8 s with a data acquisition system (DAS) sampling rate of 1230 Hz. As a result, a projection sampling rate of 984 views per gantry rotation is obtained. Theoretical, experimental, and clinical investigations have shown that, from a standpoint of aliasing, this sampling rate is near a lower limit. It is desirable to increase the scan rate to at least 0.5 s per gantry rotation to reduce motion artifacts and to reduce imaging times, but to do so would require a higher sampling rate. Hardware limitations limit maximum sampling rates, however. For example, hardware and software limitations may limit a DAS sampling rate to 1408 Hz. For 0.5 s per scan, 704 views per gantry rotation would be obtained in such a system, yielding a 28.5% reduction in the number of available views, and hence data, as compared to other CT imaging systems that provide 984 views per gantry rotation. If proper compensation is not performed, view aliasing artifacts, such as streaks, will result in reconstructed images. Radiologists object to such aliasing artifacts. In sum, when the number of views acquired per gantry rotation is too low, insufficient data results, thereby, causing objectionable image artifacts.
U.S. Pat. No. 6,285,732, issued to Hsieh on Sep. 4, 2001, and incorporated herein by reference, teaches methods and apparatus for reducing aliasing artifacts in computerized tomographic imaging using adaptive, non-uniform view interpolation within a selected view range. Additionally, in U.S. Pat. No. 6,285,732, Hsieh teaches a method of weighting the views to compensate for the non-uniform interpolation, and filtering and backprojecting the views to generate an image of the imaging object that he says reduces view aliasing artifacts “without clinically unacceptable reduction in spatial resolution.”
In practice, because view interpolation inherently results in some reduction in spatial resolution, it remains desirable to provide a method and a system for CT imaging that reduces view aliasing artifacts, without employing view interpolation and its inherent limitations.